PHOBOS is a name you are going to hear a lot in the coming years. It may be little more than an asteroid - just two-billionths of the mass of our planet, with no atmosphere and hardly any gravity - yet the largest of Mars's two moons is poised to become our next outpost in space, our second home.

Although our own moon is enticingly close, its gravity means that relatively large rockets are needed to get astronauts to and from the surface. The same goes for Mars, making it expensive to launch missions there too - perhaps even prohibitively expensive if President Obama's review of NASA's human space exploration policy is to be believed. Last October, a committee of independent experts chaired by industrialist Norman Augustine concluded that NASA faced a shortfall of around $3 billion a year if it still intends to send astronauts back to the moon - let alone Mars - by 2020. But that doesn't mean that humans have nowhere to go.

One option the Augustine report suggested would take NASA crews to nearby asteroids and to the moons of Mars. "The bulk of the cost of a Mars mission is getting people to the surface and back again," says Pascal Lee, chairman of the Mars Institute in Moffett Field, California. "If you wait for everything to be ready, it will be decades. Phobos offers us a way to get to the very doorstep of Mars."

Because Phobos is so small, the gravitational field it generates is weak, so much so that once you have established yourself in Martian orbit, landing and take-off from Phobos needs only the smallest of impulses. That means it is cheaper and easier to send spacecraft to distant Phobos than to send them to the surface of our own moon.

From Phobos we could easily explore the surface of Mars using telescopes or remote-controlled rovers before making the final descent to the planet's surface when funding allows (see "The Martian night shift").

But there is more to Phobos than just a convenient stopping-off point - much more. Phobos itself is a giant celestial mystery. "We know what all the solar system bodies that we have explored are, except for Phobos," says Lee. "We really do not know how it formed."

Phobos was discovered, along with Mars's smaller moon Deimos, in 1877 by American astronomer Asaph Hall at the US Naval Observatory in Washington DC. For most of their subsequent history, the moons' diminutive size has relegated them to mere footnotes in the astronomical textbooks. Phobos is an irregularly shaped rock just less than 28 kilometres across, while Deimos is even smaller (see diagram). So they were dismissed as being small space rocks that wandered too close to Mars and were unlucky enough to be captured by its gravity.

This view was bolstered by the first measurements of Phobos's composition, taken by the spacecraft Mariner 9 and Vikings 1 and 2 in the 1970s (see Missions to Phobos). Sunlight reflecting from the surface showed that Phobos was dark, absorbing more than 90 per cent of the incoming sunlight and resembling the meteorites known as carbonaceous chondrites. These ancient celestial objects are thought to originate in the furthest parts of the asteroid belt, twice as far from the sun as Mars itself. The most recent measurements of Phobos revealed a closer resemblance to even older asteroids found only in the outer solar system beyond the main belt. The same is true for Deimos.

Space oddity

So captured asteroids they are, then? Not quite. The orbits these moons follow are not what you would expect for captured bodies. Instead of orbiting in randomly inclined orbits, as would happen if they were seized at different times, both Phobos and Deimos follow paths that lie close to the equatorial plane of Mars. What is going on?

Equatorial orbits imply that the moons formed in situ from the same coalescing cloud that became Mars. But if this is the case, then the moons' composition makes no sense; Phobos and Deimos should resemble Martian rock, not carbonaceous chondrites. In a bid to understand the composition and thereby the origin of Phobos, the European spacecraft Mars Express has made a daring sequence of fly-bys, swooping to within 460 kilometres of the moon in 2006 and 270 kilometres in 2008.

That close, Phobos's minuscule gravity altered the spacecraft's velocity by just a few millimetres per second. Nevertheless, mission controllers on Earth succeeded in identifying its effect on the radio tracking signal - a variation of just one part in a trillion on the carrier signal.

"It was an incredible achievement on the part of everyone involved," says Martin Pätzold at the University of Cologne in Germany and the leader of the Mars Express Radio Science experiment. It allowed Phobos's mass to be measured 100 times more accurately than before, and also raised the possibility that the moon could become a proxy spacecraft for exploring Mars's internal structure (see "Probing Mars").

During the fly-bys, Mars Express's High Resolution and Stereoscopic Camera mapped the surface of Phobos, which led to the most precise 3D model of the moon so far constructed and a measure of its volume. Although it is much less certain than the mass, knowing the volume allows an average density to be calculated using the ultra-precise mass figure. What emerges is the most interesting paradox of all.

"The mean density is unexpectedly low. It must be a porous body," says Pätzold. So rather than being a single chunk of solid rock, there are probably vast caverns inside the moon, which could shelter future visitors from the ravages of space radiation.

Phobos landing

Without actual samples from the moon, though, its composition remains largely unknown. If it is a captured asteroid, the material it is made from will be less dense than ordinary rock, making the hollow fraction likely to be around 15 per cent. If the moon is made of the equivalent of Martian rocks, however, then the Phobos's void must be much higher: up to 45 per cent.

This in itself is a headache for planetary scientists. If Phobos turns out to be made of Martian rock, the size of the voids means that the moon is unlikely to have formed from tiny dust grains building up in orbit as Mars formed beneath it, as this would lead to a solid body. Instead, Pätzold and Pascal Rosenblatt of the Royal Observatory of Belgium in Brussels favour a sequence of events in which a giant impact on Mars threw large chunks of debris into orbit. These then settled against one another at haphazard angles to form the conglomeration we now call Phobos.

To test this suggestion, Mars Express will be revisiting the moon in March for its closest fly-by yet. The spacecraft will close to within a mere 60 kilometres of the barren surface, supplying the team with the first inklings of Phobos's gravity field.

"The gravity field is related to the internal distribution of mass," says Rosenblatt. So, when Mars Express is over a void it will not be pulled as hard as when it is over solid rock.

They will also be using the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument to probe inside Phobos. During previous fly-bys, the MARSIS team learned how to bounce their radar off the moon. Now they plan to use ground-penetrating radar to peer inside. "We hope to see subsurface structure in March but there are a lot of factors in play," says Andrea Cicchetti of the Italian Institute of Physics of Interplanetary Space in Rome who is part of the MARSIS team.

The team is especially keen to nail down the composition of the moon whose spectrum suggests it is a captured asteroid. Rosenblatt thinks there is a get-out clause, however. "The surface spectrum could be the result of billions of years of space weathering," he says. Without an atmosphere to protect them, the Martian rocks that coalesced to form Phobos could have been altered superficially by the charged particles they have been soaking up from the sun for billions of years, disguising their true identity and fooling the spectrometers. The solution? Land on Phobos and bring samples back for us to study here on Earth.

This is exactly what Russia plans to do in late 2011 with the Phobos-Grunt (Phobos-soil in Russian) mission. "We cannot understand the origin of Phobos without knowing what the moon is made from, and Phobos-Grunt will tell us that," says Rosenblatt.

Phobos-Grunt may even provide planetary scientists with crucial information about Mars itself. During the last four billion years, meteorite impacts with Mars will have blown debris into orbit. Phobos must have ploughed through these debris streams, some of which contained large chunks, as demonstrated by the moon's 9-kilometre-wide crater, Stickney.

Most of the impacts would have been much smaller, the probable explanation for the grooves that line the surface of Phobos. Recent mapping by Mars Express has shown that the grooves originate from the leading apex of Phobos, the point that always faces in the direction of the moon's motion and so is the natural bullseye for incoming debris.

The exciting fact is that nature has been collecting samples of Mars for billions of years and storing them on Phobos - one of the easiest places in the entire solar system for us to reach. All we have to do is go and get them. "Phobos is the Library of Alexandria for Mars," says Lee. "Samples from early Mars may be much better preserved on Phobos than on Mars itself." They may even contain the chemical signature of Martian life, though Lee puts a heavy emphasis on the "may" in that statement.

And Phobos-Grunt could just be the first in a line of increasingly ambitious missions to Mars's largest moon. "Mars should remain the ultimate destination for manned exploration," says former astronaut Leroy Chiao and member of the Augustine committee. "But if we [the committee] had asked outright for the money required to land on Mars, we would have lost credibility."

To bridge the gap, Lee envisages Phobos as an ideal stopover while techniques and equipment are developed by NASA to allow us to land on Mars. He has already studied the feasibility of a hypothetical Canadian mission to Phobos. So successfully did he make his case that Lee is now involved in a similar study for NASA.

Home from home

He points out that just getting to Phobos would allow astronauts to practise key techniques for reaching Martian orbit, such as aerobraking, in which a spacecraft loses speed by surfing the planet's atmosphere.

What's more, the moon could host a warehouse of rocket parts and other equipment, built up over time by passing robotic exploration missions. When astronauts arrive, any worn-out or malfunctioning equipment could quickly be replaced.

If the NASA mission goes ahead, it would target an amazing structure on Phobos known as the monolith. This solid slab of rock sticks upwards from the surface and extends 90 metres into space. "It's the Empire State building of Phobos," jokes Lee.

The spacecraft would land close to the monolith, so that it could study the exposed rock, then hop to another part of the moon and collect some more samples. It would then take off and fly to Deimos, to collect samples from the smaller moon. Finally, it would return to Earth. "It would be an exciting mission," says Lee. "We could fly within five years of getting a budget."

It is now in the hands of the White House, as they consider the Augustine Report. Not even Chiao has an inside track on the likely outcome of those deliberations. "Like everybody else, I'm just waiting for the administration to make up its mind about how it wants to respond," he says.

Landing on Phobos is a way of getting close to Mars. But surely it would feel like driving all the way to your destination and then not daring to knock on the door? Not according to Lee. "There are plenty of people who would go, including me," he says. "The view of Mars alone would be staggering."

Chiao, however, says he would find it tough being on a Phobos-only trip. "It's hard for me to imagine going all that way and not getting to the surface of Mars," he says. "But if it were a choice of that or nothing, I'd take Phobos any day!"